Repression of arginine anabolic genes and induction of arginine catabolic genes are mediated by a three-component protein complex, interacting with specific DNA sequences in the presence of arginine. Although ArgRI and Mcm1, two MADS-box proteins, and ArgRII, a zinc cluster protein, contain putative DNA binding domains, alone they are unable to bind the arginine boxes in vitro. Using purified glutathione S-transferase fusion proteins, we demonstrate that ArgRI and ArgRII1-180 or Mcm1 and ArgRII1-180 are able to reconstitute an arginine-dependent binding activity in mobility shift analysis. Binding efficiency is enhanced when the three recombinant proteins are present simultaneously. At physiological concentration, the full-length ArgRII is required to fulfill its functions; however, when ArgRII is overexpressed, the first 180 amino acids are sufficient to interact with ArgRI, Mcm1, and arginine, leading to the formation of an ArgR-Mcm1-DNA complex. Several lines of evidence indicate that ArgRII is the sensor of the effector arginine and that the binding site of arginine would be the region downstream from the zinc cluster, sharing some identity with the arginine binding domain of bacterial arginine repressors.